For environmental protection, honeycomb filters comprising ceramic honeycomb structures and plugs alternately sealing both inlet and outlet portions for removing carbon-based particulate matter from exhaust gas discharged from diesel engines have been finding wider use.
FIG. 12 shows the cross sectional of a conventional honeycomb filter. In a honeycomb filter 50, an exhaust gas containing particulate matter flows into flow paths 52 open at inlets 57, passes through porous ceramic cell walls 53, and exits from outlets 58 via the adjacent flow paths. The particulate matter contained in the exhaust gas is captured by pores (not shown) in the cell walls 53. When the particulate matter continues being captured in the honeycomb filter 50, the pores in the cell walls 53 are clogged, resulting in drastic decrease in capturing performance and increase in pressure loss, thereby lowering engine power. The accumulated particulate matter can be burned by an electric heater, a burner, microwaves, etc. to regenerate the honeycomb filter 50. However, because as high exhaust gas temperatures as burning PM are unlikely to be achieved under usual operating conditions of diesel engines, technologies for accelerating the oxidation of PM by catalytic materials carried by the honeycomb filter 50 to regenerate the honeycomb filter 50 have been investigated. For instance, honeycomb filters having catalysts comprising platinum-group metals and rare earth oxides such as cerium oxide, etc., which are integrally carried by high-specific-surface-area alumina, are being put to practical use. Using such catalyst-carrying honeycomb filters, combustion reactions can be accelerated by the catalysts to remove the accumulated PM.
Even if such catalyst-carrying honeycomb filters were used, however, catalysts would not be activated to burn PM off in a driving state providing low exhaust gas temperatures during traffic jam, etc. JP 2002-122015 A discloses a method for cleaning exhaust gas comprising presuming the amount of PM accumulated in a catalytic-material-carrying filter depending on the operating condition of a diesel engine, and injecting an unburned fuel into an upstream side of the filter to accelerate the oxidation reaction of the fuel on the catalytic material, thereby burning the accumulated PM. In such forced regeneration of the filter by the added fuel, the oxidation reaction of the fuel is fully activated in a downstream side where the fuel is in good contact with the catalytic material, but the catalyst is substantially at the exhaust gas temperature in the inlet-side end portion of the filter, resulting in always low activation. Accordingly, when driving conditions at low exhaust gas temperatures continue, PM is likely to be accumulated at the inlet-side end surface 57 where the catalytic material has low activity, particularly on the exhaust-gas-inlet-side end surfaces of the exhaust-gas-inlet-side plugs 54. As a result, the inlet-side the end surfaces of the inlet-side flow paths 52 in the filter are clogged, resulting in pressure loss increase.
To prevent pressure loss increase by the accumulation of PM at the exhaust-gas-inlet-side end surface of a catalyst-carrying honeycomb filter, particularly on the exhaust-gas-inlet-side end surfaces of exhaust-gas-inlet-side plugs, the applicant disclosed in JP 2004-251266 A, a honeycomb filter 80 shown in FIG. 15, and a method for cleaning an exhaust gas. The honeycomb filter 80 comprises has a catalytic material carried by at least part of porous ceramic cell walls and/or plugs of a honeycomb structure, at least one exhaust-gas-inlet-side plug 85 being separate form an exhaust-gas-inlet-side end surface 87. Because inlet-side end surfaces 85a, on which PM tends to be accumulated, are located in a high-temperature portion of in the honeycomb filter 80, the catalytic material carried by the inlet-side end surfaces 85a is activated in the forced regeneration of the filter by adding a fuel, so that PM is easily burned to prevent the clogging of the flow paths. Accordingly, the honeycomb filter does not suffer pressure loss increase for a long period of time.
JP 3-68210 B and JP 2004-19498 A disclose methods for producing honeycomb filters by forming plugs at positions separate from inlet-side end surfaces.
JP 3-68210 B discloses a first method shown in FIG. 13. As shown in FIG. 13(a), after the end surfaces of the flow paths not to be plugged are sealed by wax 66, the inlet end surface 67 of the honeycomb structure 61 is immersed in a plugging slurry 69 to introduce the slurry 69 into the flow paths 62 without wax. Because the honeycomb structure made of a porous ceramic absorbs water, an upper part of the slurry entering into the flow paths 62 is solidified by losing water to the cell walls, while a lower part of the slurry not in full contact with the water-absorbing cell walls is not solidified. This honeycomb structure is turned upside down as shown in FIG. 13(b), so that the slurry remaining in the flow paths spontaneously flows downward to the solidified slurry portion to form plugs 64. The positions of the inlet-side plugs are determined depending on the height of the slurry. Disclosed as a second method is a method of embedding ceramic chips in the honeycomb structure to form inlet-side plugs, and sintering them to integrally bond the plugs to the cell walls.
JP 2004-19498 A discloses a method of injecting a cordierite-composition paste containing an organic binder and water into a honeycomb structure at positions of 10 mm inside from an upstream-side end surface of the honeycomb structure by a paste injector (dispenser) comprising a pipe of the predetermined length, thereby alternately forming plugs.
As a method for forming plugs in a flow path end portion by supplying a plugging material by a nozzle, by which the plugs are not formed at positions separate from the inlet-side end surface, JP 5-23507 A discloses a method of injecting a plugging material into a flow path from a lower opening of a nozzle connected to a plugging material supplier in a relative movement of the honeycomb filter and the plugging material supplier, thereby forming plugs, and JP 6-39219 A discloses a method for injecting a plugging material from pluralities of nozzles to form plugs.
Referring to the methods described in JP 3-68210 B and JP 2004-19498 A, the inventors attempted the methods of forming plugs at positions separate from the end surfaces of the honeycomb structures, but failed to produce honeycomb filters because of the following problems.
In the first method disclosed in JP 3-68210 B, when the slurry 69 is charged into the flow paths 62, water is absorbed by the cell walls in both upper and lower portions of the slurry, resulting in solidification occurring simultaneously in the upper and lower portions. It is thus difficult to solidify only the upper portion of the slurry, so that the entire flow paths upstream of the inlet-side plugs are sealed by the solidified slurry. Accordingly, it is difficult to form plugs at positions separate from the inlet-side end surface.
In the second method disclosed in JP 3-68210 B, because it is difficult to provide the extrusion-molded honeycomb structure and the ceramic chips with completely the same properties such as a thermal expansion coefficient, etc., gaps are generated between the ceramic chips and the cell walls due to expansion and shrinkage by sintering, resulting in a lower particulate-matter-capturing effect, the detachment of the ceramic chip plugs from the cell walls, or the breakage of the cell walls by the ceramic chips.
In the method disclosed in JP 2004-19498 A, in which a paste is supplied to the upstream portion of the honeycomb structure inside the upstream-side end surface by a pipe-shaped injector, the pipe inserted into the flow path comes into contact with porous cell walls, resulting in the breakage of the cell walls in the flow path end portion. When the cell walls are broken, the particulate matter is not fully captured, resulting in lowered cleaning performance. Also, a cream-like ceramic powder paste is likely to clog the pipe, so that plugs cannot be formed at positions separate from the end surface.
Further, there is a problem that voids (cavities) are likely generated in the plugs. As compared with the formation of plugs in the flow path end portions, the formation of plugs in the flow paths at positions separate from the end surface is likely to generate voids, because more water in the paste is absorbed by the cell walls. Namely, when plugs are formed at the end surface, water in the paste absorbed by the cell walls moves only toward another end surface. On the other hand, when plugs are formed at positions separate from the end surface, water moves from the injected portion in the cell walls toward both end surfaces, so that much more water is absorbed. Such voids decrease the reliability of the plugs, and provide the plugs with apertures in an extreme case, failing to function as a filter. Such phenomenon is remarkable when plugs are formed, for instance, 10 mm or more separate from the end surface of the honeycomb structure.
The above phenomenon similarly occurs, even when plugs are formed at positions separate from the end surface by the method of forming plugs in the flow path end portions using the nozzle described in JP 5-23507 A and JP 6-39219 A.
In the honeycomb filter 80 described in JP 2004-251266 A, which was proposed by the inventors, when a large amount of PM is accumulated at the inlet-side end surface 85a in a low exhaust gas temperature state for a long period of time, even the forced regeneration of the filter by the injection of an unburned fuel and/or a hydrocarbon gas sometimes cannot fully elevate the temperature of the inlet-side end surface 85a, failing to make the catalyst fully active in the exhaust-gas-inlet-side plugs 85 and the upstream-side cell walls 82u, and resulting in pressure loss increase in the honeycomb filter by the remaining PM.